More and more medical displays are used as replacement for traditional film in radiology. Instead of using expensive film a radiologist looks at a digital image on a high-quality (typically greyscale) medical display. An additional advantage of the medical display is that the radiologist is able to perform image-processing operations on the medical image such as contrast enhancement, zoom . . . and this makes it easier to diagnose. It is obvious that medical displays require very high quality and quality control as they are very often used for primary diagnosis and therefore life-critical decision taking. A lot of regulations and recommendations exist. One example of such a quality requirement is the “DICOM/NEMA supplement 28 greyscale standard display function”. It describes how the greyscales in a digital medical image should be mapped to the output levels of a medical output device such as a display, a film-printer . . . in order to maximise the visibility of small details present in the digital image file.
General information with respect to medical imaging may be found in the book “Fundamentals of Medical Imaging”, by Paul Suetens, Cambridge University Press, 2002. A typical medical image as created by an imaging device (X-ray, ultrasound, scanner . . . ) contains between 256 (8 bit) and 4096 (12 bit) greyscales. However present medical viewing applications normally limit the output to 256 concurrent greyscales. The radiologist then uses window/levelling (a kind of contrast enhancement) to selectively visualise all greyscales in the original image file. Medical displays on the other hand tend to have at least 1024 (10 bit) output greyscales, therefore there are several possibilities to map the 256 greyscales from the medical image to the 1024 available greyscales from the display. Just mapping/selecting these 256 greyscales in a linear way on the 1024 display greyscales will result in loss of information: it will be impossible to visually distinct between some neighbouring greyscale levels from the medical image. This is because present medical displays, which often are LCD-displays, often have a highly irregular transfer curve that strongly differs from the traditional gamma curve of a CRT display and that is not adapted to the more or less logarithmic response of the human eye.
FIG. 1 and FIG. 2 are extracts from the document “DICOM/NEMA supplement 28 greyscale standard display function”. FIG. 1 shows the principle of changing the global transfer curve of a display system to obtain a standardised display system 102 according to a standardised greyscale standard display function. In other words, the input-values 104, referred to as P-values 104, are converted by means of a “P-values to DDLs” conversion curve 106 to digital driving values or levels 108, referred to as DDL 108, in such a way that, after a subsequent “DDLs to luminance” conversion, the resulting curve “luminance versus P-values” 114 follows a specific standardised curve. The digital driving levels then are converted by a “DDLs to luminance” conversion curve 110 specific to the display system and thus allow a certain luminance output 112. This standardised luminance output curve is shown in FIG. 2, which is a combination of the “P-values to DDLs” conversion curve 106 and the “DDLs to luminance” curve 110. This curve is based on the human contrast sensitivity as described by the Barten's model. It is to be noted that it is clearly non-linear within the luminance range of medical displays. The greyscale standard display function is defined for the luminance range 0.05 cd/m2 up to 4000 cd/m2. The horizontal axis of FIG. 2 shows the index of the just noticeable differences, referred to as luminance JND, and the vertical axis shows the corresponding luminance values. A luminance JND represents the smallest variation in luminance value that can be perceived at a specific luminance level. A more detailed description can be found in “DICOM/NEMA supplement 28 greyscale standard display function”, published by National Electrical Manufacturers Association in 1998.
A display system that is perfectly calibrated based on the DICOM greyscale standard display function will translate its P-values 104 into luminance values (cd/m2) 112 that are located on the greyscale standard display function (GSDF) and there will be an equal distance in luminance JND-indices between the individual luminance values 112 corresponding with P-values 104. This means that the display system will be perceptually linear: equal differences in P-values 104 will result in the same level of perceptibility at all digital driving-levels 108. In practice the calibration will not be perfect because, typically, only a discrete number of output luminance values (for instance 1024 specific greyscales) are available on the display system.
At present, a “DICOM-calibration” with medical display systems, which often—but not necessary—are LCD displays, is achieved as it has always been done with CRT-displays: by measuring the native transfer curve of the display, i.e. determining the luminance versus DDL, and using this curve to calculate a conversion table between P-values and DDLs. Measuring the native transfer curve of the display is done by placing a luminance measurement device with small acceptance angle in the centre of the display. A device with small acceptance angle is used because otherwise the variation of viewing angle characteristics of the display make the measurement data unreliable. With a device with a large acceptance angle, the measurement results are integrated values over a wide range of viewing angles. Such an approach works well for well-known technologies such as traditional photographic film and CRT-displays, but the specific nature of several of today's medical displays, such as e.g. LCD-displays, and by extension other fixed format displays such as plasma displays, field emission displays, electro luminescent (EL) displays, light emitting diode (LED) and organic light emitting diode (OLED) projection displays, introduces some important unsolved problems that can have a very negative effect on the DICOM-conformance and quality of medical imaging in general.
Several of these medical displays, such as e.g. LCD displays, typically have viewing characteristics which vary with viewing-angle: looking at an angle to the display significantly changes the perceived image. This phenomenon is illustrated in FIG. 3 and FIG. 4, showing the luminance intensity as a function of the horizontal and vertical viewing angle for a full-white video level and a full-black video level respectively. Points corresponding with an equal luminance output are connected for some luminance values. Not only is there a general change in perceived luminance, but also the native transfer curve of the panel changes radically when the panel is looked at an angle. It is obvious that this behaviour can cause poor DICOM-conformance even at small viewing angles, and can introduce a quality risk when diagnosis is performed by looking at a display at an angle. It is to be noted that nowadays it is normal behaviour to look at a medical display at a (small) angle when performing diagnosis, especially when displays are mounted on a wall and/or when multiple radiologists discuss a case together.
Another negative aspect of present high-quality medical displays is that they have variable luminance uniformity over the complete display area.
Especially the darker video levels typically show brighter and darker areas that can differ up to a factor 2 and more in luminance. At higher video levels the situation is somewhat better but still luminance differences of 30%-35% should be considered as normal. FIG. 5 shows an example of the distortion in percent from the mean luminance value over the complete display area for a fixed viewing angle. Also this luminance uniformity problem over the display area causes very bad DICOM-conformance. For people skilled in the art it will be obvious that especially at the darker video levels, even small luminance variations introduce a large distortion from the ideal DICOM-model.
In the past, solutions have been proposed to solve the problem of luminance non-uniformity, as can be seen from e.g. US-2002/154076, EP-1132884 and U.S. Pat. No. 5,359,342. In theory, by making the display completely uniform over its complete area and this for all video levels, the transfer curve will be also the same for all pixels. This means that there is no longer a problem of spatial DICOM-conformance. However, making the transfer curve equal for all pixels is only possible if the dark level of all display pixels is increased to the luminance value of the brightest pixel in the “fully off” state. The same principle holds for the highest video level: the maximal luminance of all pixels must be made equal and thus decreased to the luminance value of the darkest pixel in the “fully on” state. It is obvious that this will result in a display with a high black luminance and a low peak luminance and therefore a poor contrast ratio. A high contrast ratio is exactly one of the requirements of a high-quality medical display. Therefore, the existing solution of making the display completely uniform is not practical.
U.S. Pat. No. 5,359,342 furthermore describes a way to obtain a linear transfer curve for different regions in the display, without normalising the total brightness. Nevertheless, the system does not describe a method for obtaining an optimum DICOM conformance behaviour, whereby the transfer curve is adjusted to the individual variations of display pixels or zones. Furthermore, the correction provided in U.S. Pat. No. 5,359,342 is a constant correction, not taking into consideration the environmental changes or the conditions in which the display is used.
Up to today and to the best of our knowledge, no practical solution for these specific medical display characteristics with reference to DICOM-conformance are known. Until now it was only indirectly possible to improve spatial and off-axis DICOM-conformance of medical displays. The spatial problem could be improved by making the luminance more uniform but with a loss in contrast ratio as a major drawback. For the viewing-angle problems some manufacturers, sometimes not even being aware of it, used sensors with larger acceptance angle during calibration. In this way they achieved a somewhat better DICOM-conformance under small angles but a decrease in DICOM conformance for on-axis viewing.
In “Color correction in TFTLCD displays for compensation of color dependency with the viewing angle”, 2002 SID international symposium digest of technical papers, Boston, Mass., May 21-23, 2002, SID international symposium digest of technical papers, San Jose, Calif.: SID US, vol. 33/2, May 2002 (2002-05), pp. 713-715, G. Marcu et al. describe a method for compensation of a pixel colour variation relative to a single viewer position. The method determines the colour correction required for each pixel of a screen, such that a single viewer for a given position can see the colour unaffected by the viewing angle differences to the screen. The colour correction can be recomputed automatically as the viewer position changes, as long as the position is known.